Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION: The main function of the spine is to generate sufficient stiffness to complete a given task. As a result, most back pain is thought to be mechanical in nature, arising from problems in the structures and functions that modulate stiffness. Unfortunately, clinicians cannot readily identify these mechanical issues in any given person. As a result, treatment for back pain is a process of trial and error which generates enormous expense, unresolved pain, and persistent disability. To address this gap, Magnetic Resonance Elastography (MRE) can quantify tissue stiffness, but MRE has not yet been optimized for regular use in the spine. Here we provide a proof-of-concept using a newly developed Tomo Magnetic Resonance Elastography (TMRE) system capable of generating multifrequency displacements in deep spinal tissues using a dual-actuator setup and then quantify tissue stiffness of the intervertebral disc (IVD) in three persons with progressively increasing degeneration.

METHODS: In three males ages 32, 50 and 60, sagittal T2 Dixon MRI images were obtained from a clinical 3T MRI system (Magnetom Prisma; Siemens, Germany) using a 18Ch-bodyflex coil. The 3-D wave fields were acquired by spin-echo planar imaging with flow-compensated motion-encoding gradient. Eight wave-phase offsets were recorded for each of the three motion directions. Four vibration frequencies were applied: 60, 70, 80 and 90Hz using two pneumatic actuators placed bilaterally on the paravertebral muscles and centered at the third lumbar level. TMRE data were post-processed using multifrequency wave-number recovery (k-MDEV) inversion algorithm available online (https://bioqic-apps.charite.de) The resulting shear wave speed (SWS) values were used as a surrogate of tissue stiffness and then compared to Pfirrmann grading of disc degeneration (1-5) performed by an experienced MRI spine researcher.

RESULTS: Figure 1 shows the resulting SWS maps from the three participants.  Pfirrmann grading results were as follows: Grade I, 0 IVDs; Grade II, 5 IVDs; Grade III, 4 IVDs, Grade IV, 5 IVDs; Grade V, 1 IVD. Figure 2 demonstrates a negative relation between increasing SWS and progressive IVD degeneration (Spearman’s rank correlation coefficient ρ = -0.792, p < 0.001)

DISCUSSION: Our results are the first to be presented from the spine using a multi-frequency dual-actuator setup and the k-MDEV inversion algorithm. The resulting data suggest the feasibility of using TMRE to produce high-resolution elastography results over multiple frequencies in deep spinal tissues. Our data is congruent with prior MRE studies¹ and mechanical IVD testing² that demonstrate an inverse relation between the severity of IVD degeneration and stiffness. In contrast, a recent study using a single-frequency MRE setup has shown a direct relation between increasing degeneration and increasing stiffness³. Although these contradictory results have yet to be reconciled, preliminary discussions suggest an effect of post-processing decisions.

CONCLUSION: TMRE with k-MDEV inversion allows measurement of in vivo mechanical properties of IVDs and may provide additional information in disc degeneration beyond standard morphological changes. Prior to the clinical use of this technique, future studies should be conducted to evaluate the reproducibility and repeatability of spinal TMRE in the spine and in particular, its potential confounders.